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This invention relates to methods of efficiently applying low temperature heat to absorption refrigeration cycles and absorption power cycles. In conventional absorption cycles, high temperature heat is applied to a high-pressure desorber or generator, where high-pressure vapor is desorbed from the absorbent solution. When the resulting vapor is pure refrigerant, as with LiBrxe2x80x94H2O absorption cycles, no further treatment is necessary. When the resulting vapor has appreciable absorbent content, as with NH3xe2x80x94H2O absorption cycles, it is necessary to distill, analyze, or rectify the vapor to higher refrigerant purity by contacting it with lower temperature absorbent. That distillation may be done either adiabatically or diabatically. The external heat addition portion of the desorber is customarily termed the generator, and the distillation portion may have internal heat addition.
When the external heat source is at relatively low temperature, for example only modestly above the generator temperature, and when it has a temperature glide, then very little of the heat content of the source can be effectively transferred to the generator using conventional techniques. Consider for example a combustion exhaust stream at 270xc2x0 C., and an absorption cycle generator at 170xc2x0 C. Given a 30xc2x0 C. minimum temperature difference for heat transfer, it is only possible to cool the heat source from 270xc2x0 C. to 200xc2x0 C. by transferring heat to the generator. This is only on the order of 30% of the available heat content of that source.
Two other possible problems arise when supplying low temperature waste heat such as combustion exhaust gas to an absorption cycle. With one approach, the combustion exhaust directly contacts the heat transfer surface of the generator. However, there are usually stringent limitations on the allowable pressure drop of the exhaust gas. For example, the backpressure for a combustion turbine is typically specified at no more than six to ten inches water column. The generator which satisfies both this criterion and also the specialized mass transfer criteria of the absorbent solution will be very large and costly. That is, the transfer geometry necessary for effective desorption is very different from that necessary for low xcex94p extraction of heat from combustion gas. Alternatively a closed cycle heat transfer fluid can be circulated between the heat source and the generator, such that the geometry of each heat exchanger is free to be optimized for the respective requirements. This has the disadvantage that two separate heat exchanger temperature differentials are interposed between the waste heat and the absorbent solution in the generator. For example, the heat transfer fluid must be heated to well above the generator peak temperature. If water is the heat transfer fluid, it will have to be at a much higher pressure than the generator.
There are a variety of hydrocarbon-fueled prime movers which exhaust a combustion gas, including gas turbines, microturbines, reciprocating engines, and fuel cells. Depending upon the prime mover, the exhaust temperature varies from 200xc2x0 C. to 550xc2x0 C. There is increasing need and desire to convert that exhaust heat to useful purpose, such as cooling, refrigeration, shaft power, or electricity. It is one objective of the present invention to convert greater fractions of waste heat to useful purpose than has heretofore been possible. It is another objective to avoid the prior art disadvantages of applying waste heat to absorption cycles, i.e., the high backpressure associated with direct contact heat transfer, and the high temperature differentials associated with pump-around loops. That is, there is a need for a method of transferring heat from a low temperature sensible heat source to an absorption cycle which avoids the xcex94p and xcex94T and high pressure penalties associated with traditional methods, while achieving greater utilization of the heat source, i.e., more useful result.
The above and other useful objects are achieved by apparatus wherein thermal energy is converted into at least one of refrigeration, cooling, and shaft power comprising:
a) an absorbent solution comprised of sorbate plus absorbent;
b) a desorber comprised of:
i) an entry port for sorbate-rich liquid absorbent;
ii) a means for separating said sorbate-rich absorbent into sorbate vapor and sorbate-lean absorbent;
iii) an exit port for said sorbate vapor; and
iv) an internal heat exchanger which has an entry port in communication with said sorbate-lean absorbent;
c) an external heat exchanger which is in thermal contact with said thermal energy;
d) a first flowpath from an exit port of said internal heat exchanger to said external heat exchanger; and
e) a second flowpath from said external heat exchanger to said desorber;
and also by process comprising:
a) circulating an absorbent solution successively through absorbing and desorbing steps;
b) desorbing the absorbent solution into high-pressure sorbate vapor and heated strong absorbent by heating it;
c) using the heated strong absorbent as the heating agent in step b);
d) reheating said heating agent by thermally contacting it with said thermal energy; and
e) combining said reheated heating agent with said heated strong absorbent.
The greater utilization of the thermal energy in the waste heat or other low temperature heat source is accomplished by applying it to a heat transfer agent, and then applying the heat transfer agent heat to at least part of a distillation step, (when present) which is at lower temperature, and/or by applying it to an intermediate-pressure desorber which is at lower temperature. Either or both of these steps further reduce the heat transfer agent temperature to below the high-pressure generator temperature, and in turn make it possible to reclaim lower temperature heat from the heat source. With this technique, the heat transfer agent can be routinely cooled to approximately 80xc2x0 C. or lower, which means the combustion gas can be cooled to approximately 100xc2x0 C. or lower.